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30 watt +20 db lineair met mrf 237 & mrf 238 fm/cw.

Click for enlarging this design:
Here a design from the Motorola engineering Bulletin.

This schematic was built already several times by several amateurs with good results.
However, this end-stage is not suitable for S.S.B or A.M. because we are using the transistors in class C!

This schematic can be built on a double-sided print of 6 X, held with a purchaser, 10 mm.
The end-stage works with the VHF transistor MRF 237 in TO-39 a housing, the emitter has been put through with the housing, (these are soldered in the print to the back of the print and has no further cooling necessary.).
All self-inductions with few exceptions (the choke inductors and L5) have been carried out as stripline (therefore a component of printed wiring).
Under some of the printed wires the conductor layer has been removed to reduce losses.

The amplifier: the MRF 238 is a 30 Watt power transistor for V.H.F. with 10 Db power gain.
Very important in this type of design is sufficient mass as a print surface.
At the most important places, the copper must be soldered above and below (don't use too thin wire.)

The capacitors that C7 forms must be assembled as closely as possible to the base connection of the MRF 238 transistor.
You must use loss arm, for example, ceramic chip capacitors and quality trimmers for example ARCO.
Or use air trimmers suitable for VHF.
The MRF 238 transistor must be assembled on a cooling plate.

A PI filter is used for oppressing harmonics.
Advisable to place between the end-stage still an extra filter.
So the oppression of the harmonics will be a -35 Db better still further.

Much attention in this design has been given to oppressing the notorious spurious oscillations.
By good HF separation of both transistors, disconnecting the feeding lines, and the way they have been dimensioned.

Components 30 VHF FM 2-meter band Watt Lineair.

Q1 MRF 237 RFC 2 10 turns. No. 20 AWG ,Enameled on 470-ohm ,1/2 W Resistor with Ferrite Bead Ferroxcube 5659065/48
Q2 MRF 238
C1 0.001 uF 50 V Erie Disc RFC 3 10 turns No 20 AWG, enameled on 270 -ohm 1/2 W resistor with Ferrite Bead Ferroxcube 5659065/48
C2 39 pF Dipped MICA RFC 5 Ferroxcube VK20020/4B
C3 24 pF Dipped MICA L1 (3.30 X 0.254 cm)Airline inductor
C4 91 pF Dipped MICA L2 (1.52 X 0.254 cm ) Airline inductor
C5 8 - 60 pF ARCO 404 Trimmer L3 Base Lead Q1
C6,C10 15-115 pF ARCO 406 Trimmer
C7 250 pF and 100 pF UNELCO in Parallel L4 Collector Lead Q1
C8,12,14 0.1 uF ,50-Vdc ,Erie Disc
C9 82 pF Dipped MICA L5 Airline Inductor and 1-turn No .18 AWG   (0,64 cm) I.D.
C11 500 pF Dipped MICA L6 (4,83 X 0,254 cm ) Airline inductor
C13 0,018 uF Chip L7 (1,78 X 0,254 cm ) Airline inductor
C15 1 uF 35 Vdc Tantalum L8 (2,29 X 0,254 cm ) Airline Inductor
C16 680 pF Allen-Bradley Feedthrough L9 (3,56 X 0,254 cm) Airline Inductor
C17 100 pF Dipped MICA L10 10 turns ,No 20 AWG ,Enameled Wire (0.59 cm) I.D.
R1 150 Ohm 1/2 Watt (carbon resistor !)
RFC 1,4 0,15 uH Molded Choke with Ferrite Bead  Ferroxcube 5659065/4B
The component values are rather critical remember it's VHF Stuff.

Print Layout linear below: Click for enlarged images!


Above the double-sided epoxy prints layout (6 * 10 cm dimensions).

Tunning of this amplifier:
If the complete circuit has been built.
And you have a stabilized power supply, a suitable dummy load, and a power/SWR meter attached to the linear.
Use a stabilized power supply capable of delivering contiously 5 amps at 13.8 volts.
After you connected a 2-meter FM transmitter with circa 300 milliwatt output.
You can begin tuning the RF power amplifier:

Tune the trimmers to maximum power output with the best SWR. (with a trim screwdriver of plastic or ceramics, therefore, no metal screwdriver!)
The input should be a quarter and up to half Watt. (More capacity can blow up the input transistor.)
At 300 mW the maximum capacity of 30 Watt is already reached.

When the amplifier delivers the maximum power of 30 watts with the minimum current draw and good SWR.
And works stable without spurious oscillations you can safely connect the end-stage to a well-matched antenna.

Success with de design!


Posted by beheerder on 07. August 2009 21:24 Posted by beheerder on 07. August 2009 21:24 0 Comments 14030 Reads Print
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